The present invention relates to a vehicle cruise control system incorporating a facility to automatically reduce the set speed of the controlled vehicle. The set speed represents an adjustable speed which is the speed the controller will attempt to keep the vehicle at.
Vehicle cruise controllers are becoming increasingly sophisticated. Known controllers, such as that described in EP0612641A incorporate a radar detector in order to identify the distance to and relative speed of a vehicle travelling in front (hereinafter the target vehicle) of the controlled vehicle. If the target vehicle is travelling faster than the set speed of the controlled vehicle, then the controlled vehicle continues at its set speed. However, if the target vehicle is travelling slower than the set speed, the controlled vehicle reduces its own speed to maintain a desired range behind the target vehicle. This reduces the need for frequent driver intervention to maintain a safe distance behind other vehicles. EP 0716149 discloses a further development in which the cruise control system monitors multiple target vehicles, both directly ahead of the controlled vehicle and to lanes either side of the controlled vehicle in order to ensure that the controlled vehicle does not perform an illegal overtaking manoeuvre.
The highly advanced cruise control systems as described hereinabove are predominantly fitted to luxury and high performance vehicles capable of rapid acceleration. The applicant has realised that there are circumstances in which it is desirable to reduce the set speed or a maximum acceleration limit. Such a circumstance may arise in heavy traffic, at traffic lights or queuing for road works when the controlled vehicle may travel slowly or come to a complete stop. Under such circumstances the driver""s attention may become directed elsewhere, and if the target vehicle moves out of a path of the controlled vehicle, the controlled vehicle may accelerate rapidly in order to attain the set speed. This could be disconcerting and possibly dangerous.
According to a first aspect of the present invention, there is provided a cruise controller having a value reducing means for reducing at least one of the set speed and acceleration limit when the cruise controller is controlling the controlled vehicle""s speed to maintain a predetermined distance behind a target vehicle travelling slower than the set speed of the controlled vehicle.
It is thus possible to provide a cruise controller which automatically reduces the controlled vehicle""s set speed and/or maximum acceleration limit when the speed of the controlled vehicle becomes constrained by a slower moving target vehicle. Thus, if the target vehicle subsequently moves out of a path of the controlled vehicle, the controlled vehicle will no longer accelerate so rapidly, thereby giving the driver time to adjust to the change of circumstance.
Preferably the set speed or maximum acceleration may be reduced to zero if the controlled vehicle comes to a halt. The driver is then required to signal to the cruise controller that he wishes cruise control to be resumed. A switch or some other driver operated control may be provided. Preferably, the position of the accelerator pedal is monitored such that movement of the pedal by the driver is taken to indicate that cruise control should be resumed. Use of the accelerator pedal in this manner feels natural to the driver.
Preferably the controlled vehicle""s speed is compared with the set speed and the value reduction is implemented when the magnitude of the speed difference exceeds a predetermined speed difference. The predetermined speed difference may be a set value or may be derived as a function of the set speed.
Preferably the set speed is reduced by a predetermined amount when the difference between the vehicle""s speed and the set speed exceeds the predetermined speed difference for more than a predetermined period of time. The time period and step size of the set speed reduction can be freely chosen, and may be selected such that a continuously varying function of set speed with respect to time can be simulated. Additionally or alternatively the maximum allowed acceleration can be varied in a similar manner.
Preferably the vehicle""s acceleration limit is reduced if the difference between the vehicle""s speed and the set speed exceeds a second predetermined speed difference. The second predetermined speed difference may be equal to the first predetermined speed difference. However, the first and second predetermined speed differences may be set to different values, such that the maximum acceleration threshold may be reduced before a reduction occurs in the set speed, or vice versa.
It is thus possible to provide an adaptive cruise controller for controlling a speed of a vehicle, wherein the cruise controller is responsive to data representing the vehicle speed and at least the distance to a first target vehicle ahead of the controlled vehicle, and wherein the cruise controller operates to maintain the controlled vehicle""s speed below a selectable maximum speed in the event that the first target vehicle is travelling below the selectable maximum speed, and to maintain a predetermined distance behind the first target vehicle, characterised in that when the controlled vehicle is travelling behind the first target vehicle at a speed which is slower than the selectable maximum speed, at least one of the selectable maximum speed and maximum acceleration rate is reduced:
According to a second aspect of the present invention, there is provided a method of controlling the speed of a vehicle fitted with a cruise control system arranged to follow a target vehicle at a predetermined distance when the target vehicle is travelling slower than a set speed, the method comprising the step of reducing at least one of the set speed and a maximum acceleration value when the vehicle speed is constrained to be less than the set speed because of the presence of a target vehicle travelling at less than the set speed.
According to a third aspect of the present invention there is provided a computer program product for causing a data processor within a cruise controller to implement the method according to the second aspect of the present invention.